Wet chemical treatment to form a thin oxide for high k gate dielectrics

ABSTRACT

Described herein are methods of forming a thin silicon dioxide layer having a thickness of less than eight angstroms on a semiconductor substrate to form the bottom layer of a gate dielectric. A silicon dioxide layer having a thickness of less than eight angstroms may be formed by two different methods. In one method, a sulfuric acid solution is applied to a semiconductor substrate to grow a silicon dioxide layer of less than eight angstroms. The growth of the silicon dioxide layer by the sulfuric acid solution is self-limiting. In another method, a hydrogen peroxide containing solution is applied to a semiconductor substrate for a time sufficient to grow a silicon dioxide layer having a thickness of greater than eight angstroms and then applying an etching solution to etch the silicon dioxide layer down to a thickness of less than eight angstroms.

RELATED APPLICATIONS

The present divisional application is related to, incorporates byreference and hereby claims the priority benefit of the following U.S.patent applications, assigned to the assignee of the presentapplications: U.S. patent application Ser. No. 11/052,160, filed Feb. 7,2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor fabricationand more particularly forming high dielectric constant gate dielectricsfor a transistor gate.

2. Discussion of Related Art

High dielectric constant (k) gate dielectric layers are valuable inscaling down the dimensions of transistors. This is because, in order tomake the gate dielectric layer thinner and smaller in area while stillmaintaining a high capacitance for the transistor, the dielectricconstant of the gate dielectric layer must be increased. A highdielectric constant material is defined as a material having adielectric constant greater than approximately 4 (the dielectricconstant of silicon dioxide). High dielectric constant materials includeHfO₂, Si₃N₄, Ta₂O₃, and PZT (PbZrTiO₃). Ideally, a high dielectricconstant material would be deposited directly onto a semiconductorsubstrate to form a gate dielectric layer. But, the atomic layerdeposition (ALD) process that is used to deposit high dielectricconstant materials, such as HfO₂, onto a semiconductor substrate cannotdeposit the high dielectric constant materials directly onto asemiconductor substrate, such as silicon. High dielectric constantmaterials may be deposited by ALD onto an oxide surface, such as silicondioxide. Therefore, by forming an oxide surface on a semiconductorsubstrate, the high dielectric constant material may be deposited ontothe oxide surface of the semiconductor substrate. But, silicon dioxidehas a relatively low dielectric constant as compared to the highdielectric constant materials such as HfO₂. Silicon dioxide has adielectric constant of approximately 4, and HfO₂ has a dielectricconstant of approximately 20. The overall dielectric constant of a gatedielectric formed of silicon dioxide and a high dielectric constantmaterial will be lower than that of the high dielectric constantmaterial alone. Therefore, the silicon dioxide layer must be as thin aspossible, and ideally only as thick as a monolayer of silicon dioxide (2angstroms) to minimize the effect that the silicon dioxide has oflowering the overall dielectric constant of the gate dielectric layer.

SUMMARY OF THE INVENTION

Described herein are methods of forming a thin silicon dioxide layerhaving a thickness of less than eight angstroms on a semiconductorsubstrate to form the bottom layer of a gate dielectric. A silicondioxide layer having a thickness of less than eight angstroms may beformed by two different methods. In one method, a sulfuric acid solutionis applied to a semiconductor substrate to grow a silicon dioxide layerof less than eight angstroms. The growth of the silicon dioxide layer bythe sulfuric acid solution is self-limiting. In another method, ahydrogen peroxide containing solution is applied to a semiconductorsubstrate for a time sufficient to grow a silicon dioxide layer having athickness of greater than eight angstroms and then applying an etchingsolution to etch the silicon dioxide layer down to a thickness of lessthan eight angstroms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 f illustrate a cross-sectional view of a substrate on whicha silicon dioxide layer is formed according to an embodiment of thecurrent invention to form part of a gate dielectric of a transistor.

FIG. 2 is a flow chart of the methods of forming a silicon dioxide layeron a semiconductor substrate according to embodiments of the currentinvention.

FIG. 3 is an illustration of a cross-sectional view of a single waferprocessing apparatus.

FIGS. 4 a and 4 b illustrate embodiments of different termination groupson the surface of a silicon dioxide layer.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following description numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Oneof ordinary skill in the art will understand that these specific detailsare for illustrative purposes only and are not intended to limit thescope of the present invention. Additionally, in other instances,well-known processing techniques and equipment have not been set forthin particular detail in order to not unnecessarily obscure the presentinvention.

Described herein are methods of forming a thin silicon dioxide layerhaving a thickness of less than eight angstroms on a semiconductorsubstrate to form the bottom layer of a gate dielectric. Thesemiconductor substrate may be, for example, silicon, silicon germanium,or germanium. A silicon dioxide layer having a thickness of less thaneight angstroms may be formed by two different methods. In one method, asulfuric acid solution is applied to a semiconductor substrate to grow asilicon dioxide layer of less than eight angstroms. The growth of thesilicon dioxide layer by the sulfuric acid solution is self-limiting. Inanother method, a hydrogen peroxide containing solution is applied to asemiconductor substrate for a time sufficient to grow a silicon dioxidelayer having a thickness of greater than eight angstroms and thenapplying an etching solution to etch the silicon dioxide layer down to athickness of less than eight angstroms.

In FIG. 1 a, and at block 210 of FIG. 2, a semiconductor substrate 110is provided. This semiconductor substrate 110 may be a monocrystallinesilicon substrate. A silicon dioxide layer 105 may be present on thesurface of the semiconductor substrate 110. This silicon dioxide layer105 may be a native oxide having a thickness in the approximate range of8 angstroms and 10 angstroms that would be too thick to be used as thebase for a high k dielectric gate electrode and should therefore beremoved to form a thinner silicon dioxide layer to serve as part of ahigh-k gate dielectric. The native silicon dioxide layer is removed atblock 220 of FIG. 2 to form the semiconductor substrate illustrated inFIG. 1 b. To remove the silicon dioxide layer 105 the semiconductorsubstrate 110 may be placed in a single substrate processing apparatus300 such as the one illustrated in FIG. 3. FIG. 3 is an illustration ofone embodiment of a horizontal spinning single wafer cleaning apparatus300. As shown in FIG. 3, the single wafer cleaning apparatus 300 may becontained within a housing 301. The single wafer cleaning apparatusincludes a wafer holding bracket 310. Once the semiconductor substrate308 is placed onto the bracket 310, the bracket 310 may be lowered to aprocess position as illustrated. This process position may place thesemiconductor substrate 308 a short distance above a circular plate 302.The circular plate 302 can contain transducers 304 that are capable ofemitting sound in the megasonic frequency range. A fluid feed port 316can be added to the transducer plate 302 to fill an approximate 3millimeter (mm) gap 318 between the transducer plate 302 and the bottomside of the semiconductor substrate 310 with a liquid 322 at varioustimes during processing of the semiconductor substrate 310. The liquid322 can act as a carrier for transferring megasonic energy onto asemiconductor substrate bottom surface 325 or as a way to heat the waferif the liquid 322 is hot. At least one nozzle 314 can be positioned todirect flow of a gas, vapor, or a liquid onto the top semiconductorsubstrate surface 330. In one embodiment of the invention, the nozzle314 may dispense a liquid solution to contact the top semiconductorsubstrate surface 330. The entire top semiconductor substrate surface330 may be covered with a liquid solution by dispensing a liquidsolution onto a horizontally positioned semiconductor substrate 308. Thesemiconductor substrate 308 may be a wafer having any diameter, but may,for example, have a 300 mm diameter or a 450 mm diameter. Thesemiconductor substrate 110 may be monocrystalline silicon wafer 308placed on wafer holding bracket 310.

The silicon dioxide layer 105 may be removed with a hydrofluoric acid(HF) rinse. The HF rinse may be approximately 0.5% by weight HF indeionized water and the rinse may have an etch rate of the native orsacrificial silicon dioxide layer 105 of approximately 20angstroms/minute. The HF rinse may be dispensed onto the wafer 308 as aspray 320 or as a straight dispense to form a layer of the solution 322on the surface of the wafer 308. The HF rinse may be dispensed onto thewafer 308 while the wafer is spinning. The HF rinse does not etch thesilicon substrate 110 and may therefore be applied to the siliconsubstrate 110 for a time sufficient to remove all native or sacrificialsilicon dioxide 105. The HF rinse may then be removed from the siliconsubstrate 110 with a DI-water rinse and by spinning off the rinses at aspin rate sufficient to spin off the liquid. The semiconductor substrate110 may be the silicon wafer 308. The silicon wafer 308 may remain inthe single substrate processing tool 300 after the native silicondioxide layer 105 is removed.

A silicon dioxide layer 140 may then be grown on the pure siliconsurface of the semiconductor substrate 110, as illustrated at FIG. 1 c.The silicon dioxide layer 140 may be grown to have a thickness of lessthan 8 angstroms at block 230 or it may be grown to have a thickness ofgreater than 8 angstroms at block 240 and then etched back to athickness of less than 8 angstroms at block 250. To grow the silicondioxide layer 140 to a thickness of less than 8 angstroms a sulfuricacid solution is applied to the semiconductor substrate 110. Thesulfuric acid solution may be in the approximate range of 95% to 100%sulfuric acid. A high concentration sulfuric acid solution is usedbecause it provides the best oxidation of the silicon surface. When thesulfuric acid solution is not 100% sulfuric acid, the balance of thesolution is water. In one particular embodiment the sulfuric acidsolution may be 98% by weight sulfuric acid and approximately 2% byweight water. At block 230 in the first embodiment, the sulfuric acidsolution is applied to the silicon substrate. The sulfuric acid solutionmay be dispensed as a spray 320 onto the spinning wafer 308 having apure silicon surface. During dispensation of the sulfuric acid solutionthe wafer 308 may be spinning at a rate in the approximate range of 5rpm and 50 rpm. The sulfuric acid solution may be applied to the siliconsubstrate for a time sufficient to grow a silicon dioxide layer having athickness of less than 8 angstroms. The amount of time that the sulfuricacid solution is applied to the semiconductor substrate 110 may be inthe approximate range of 30 seconds and 50 seconds. The growth of thesilicon dioxide layer by the high concentration sulfuric acid solutionis self-limiting, meaning that growth of the silicon dioxide layer willstop once it reaches a thickness of less than 8 angstroms regardless ofwhether the high concentration sulfuric acid solution is still incontact with the silicon substrate. Therefore, the sulfuric acidsolution may be left on the wafer 308 indefinitely, even after thesilicon dioxide layer has been grown. In one embodiment the growth ofthe silicon dioxide may stop at a thickness in the approximate range of2 angstroms (a monolayer) and 4 angstroms. It is valuable that thethickness of the silicon dioxide layer 140 be as thin as possible sothat it has a minimal effect on lowering the overall dielectric constantof the gate electrode. The sulfuric acid solution may be spun off of thewafer 308 at a spin rate in the approximate range of 100 rpm and 300rpm. An ammonia hydroxide rinse may then be applied to the silicondioxide layer to further remove any remaining sulfuric acid residuesthat may continue to build the silicon dioxide layer. Deionized watermay then be applied to the surface of the wafer 308 and the surface ofthe wafer 308 may be dried by spinning the wafer 308 at a spin rate inthe approximate range of 750 rpm and 1000 rpm.

At block 240, in the second embodiment, a solution containing hydrogenperoxide such as an SC1 solution (NH₄OH, H₂O₂, and H₂O) or an SPMsolution (sulfuric peroxide mixture), is applied to the semiconductorsubstrate 110 to grow a silicon dioxide layer to a first thickness ofgreater than 8 angstroms. The solution containing hydrogen peroxide maybe applied to the semiconductor substrate 110 in a single substrateprocessing tool 300 by dispensing the solution containing hydrogenperoxide onto a spinning wafer 308 as a spray 320 or as a simpledispense. After growing the silicon dioxide layer 140 to a firstthickness of greater than 8 angstroms and more particularly toapproximately 10 angstroms in thickness, the sulfuric acid solution maybe spun off of the wafer 308. Because the growth of the silicon dioxidelayer levels off at about a thickness of 8 angstroms to 10 angstroms inthe presence of the hydrogen peroxide containing solution, it may not benecessary to immediately remove the hydrogen peroxide containingsolution and rinse the silicon dioxide surface. The hydrogen peroxidecontaining solution may be removed by rinsing the silicon surface with adeionized water rinse dispensed from the nozzle 314 and then spun off ofthe wafer 308. Deionized water may applied to the surface of the wafer308 and the surface of the wafer 308 may then be dried by spinning thewafer 308 at a spin rate in the approximate range of 750 rpm and 1000rpm.

At block 250 the silicon dioxide layer 140 is etched back with a mixtureof sulfuric acid and hydrofluoric acid (HF). The mixture of sulfuricacid and I-IF may be applied to the semiconductor substrate 110 on wafer308 within the single substrate processing apparatus 300 as a spray 320from nozzle 314 to form a layer of the mixture 322. The mixture ofsulfuric acid and HF may be applied to the silicon dioxide layer untilthe thickness of the silicon dioxide layer 140 remaining is less than 8angstroms and more particularly in the approximate range of 2 angstroms(a monolayer) and 4 angstroms. The time that the mixture of sulfuricacid and HF is applied to the silicon dioxide layer may be determined bythe etch rate of the mixture depending on the concentration of the HF inthe mixture. The greater the concentration of HF, the greater the etchrate. The etch rate of the silicon dioxide layer 140 by the mixture ofsulfuric acid and HF may be in the approximate range of 0.1angstroms/second and 10 angstroms/second. The mixture of sulfuric acidand HF may be in the approximate range of 90%-98% by weight sulfuricacid, and 0.1-10% by weight HF. More particularly, the mixture ofsulfuric acid and HF may be approximately 98% by weight sulfuric acid,approximately 1% by weight HF, and approximately 1% by weight water andthe etch rate of the silicon dioxide layer 140 by the mixture ofsulfuric acid and HF may be approximately 1 angstrom/second. Once thesilicon dioxide layer 140 is etched back to the target thickness, themixture of sulfuric acid and HF is immediately spun off of the wafer 308by spinning the wafer at a spin rate sufficient to completely remove themixture and stop the etching of the silicon dioxide layer. A deionizedwater rinse may be applied to the silicon dioxide layer to remove anyremaining etching solution.

As illustrated in FIG. 4 a, the silicon dioxide layer 140 may beterminated by oxygen bridges 400 after it is grown on the semiconductorsubstrate 110. The oxygen bridge 400 termination of the silicon dioxidelayer 140 is not an ideal termination for the deposition of a high kdielectric layer 150 by atomic layer deposition. The atomic layerdeposition of a high k dielectric layer 150 may be optimized byterminating the surface of the silicon dioxide layer 140 with ammoniumgroups 410 and/or hydroxide groups 420 prior to the deposition of thehigh k dielectric layer. Additionally, terminating the surface of thesilicon dioxide layer 140 with ammonium groups 410 and/or hydroxidegroups 420 may result in a higher overall dielectric constant for thegate dielectric than the oxygen bridge termination 400. FIG. 4 billustrates the silicon dioxide layer 140 after termination of thesilicon atoms at the surface by ammonium groups 410 and hydroxide groups420. To terminate the silicon atoms of the silicon dioxide layer 140with ammonium groups 410 and hydroxide groups 420 an ammonium hydroxiderinse may be applied to the silicon dioxide layer 140 at block 260 ofFIG. 2. The ammonium hydroxide rinse may be a low concentration ofammonium hydroxide diluted in water. The ratio of ammonium hydroxide towater in the ammonium hydroxide solution may be in the approximate rangeof 750:1 and 250:1, and more particularly the ratio of ammoniumhydroxide to water may be 500:1. This ammonium hydroxide solution may beapplied to the surface of the silicon dioxide layer 140 in the singlesubstrate processing apparatus 300. The ammonium hydroxide solution maybe dispensed on the silicon dioxide layer 140 on wafer 308 as a spray320 or as a simple dispense from nozzle 314 while the wafer 308 isspinning at a rate in the approximate range of 5 rpm and 50 rpm. Theammonium hydroxide solution 322 may be left on the surface of thesilicon dioxide layer 140 for a time sufficient to terminate all of theoxygen bridges 400 on the silicon dioxide layer 140 with ammonium groups410 or hydroxide groups 420. Most of the silicon atoms of the silicondioxide layer will be terminated by hydroxide groups 420 andapproximately 1%-10% of the silicon atoms may be terminated by ammoniumgroups. The percentage of the silicon atoms on the silicon dioxide layerthat will be terminated with ammonium groups is higher with a greaterconcentration of ammonium hydroxide in the solution. The ammoniumhydroxide solution also serves to remove sulfuric acid residues from thesurface of the silicon dioxide layer 140. Because of the limited numberof silicon atoms that may be terminated at the surface of the silicondioxide layer, the termination of the silicon atoms is also aself-limiting reaction. The ammonium hydroxide solution may be removedfrom the surface of the wafer 308 by spinning the wafer at a spin ratein the approximate range of 100 rpm and 300 rpm.

The wafer 308 having the silicon substrate 110 may then be removed fromthe single wafer processing apparatus 300 and placed in an atomic layerdeposition chamber. There, by atomic layer deposition, a layer of high kdielectric material 150 is deposited over the silicon dioxide layer 140.Atomic layer deposition (ALD) utilizes pairs of precursor gas pulses todeposit a film one monolayer at a time. ALD is therefore a method wherethe exact thickness of the film being formed may be controlled, which isvaluable in forming a high k dielectric material. In one particularembodiment, the high k dielectric material may be halfnium oxide (HfO₂)that has a dielectric constant of approximately 20. A gate electrodematerial 160 is then formed over the high k dielectric material 150. Thegate electrode material 160 may be polysilicon, metal, or a combinationof polysilicon and metal. The gate electrode material 160, the high kdielectric layer 150, and the silicon dioxide layer 140 may then beetched to form a transistor gate 100 illustrated in FIG. 1 f. Thetransistor gate is formed of the gate dielectric 130 and the gateelectrode 160. The semiconductor substrate 100 may then have source anddrain regions 120 formed by an implant of dopants that is self-alignedwith the transistor gate structure.

It is to be appreciated that the disclosed specific embodiments are onlymeant to be illustrative of the present invention and one of ordinaryskill in the art will appreciate the ability to substitute features orto eliminate disclosed features. As such, the scope of the Applicant'sinvention is to be measured by the appended claims that follow.

1. A device, comprising: a semiconductor substrate; a pair of source anddrain regions formed within the semiconductor substrate; a compositegate dielectric layer formed above the semiconductor substrate and thepair of source and drain regions, comprising a silicon dioxide layerhaving a thickness of less than approximately 8 angstroms and a highdielectric constant material; and a gate electrode formed above thecomposite gate dielectric layer.
 2. The device of claim 1, wherein thesilicon dioxide layer has a thickness in the approximate range of 2angstroms and 4 angstroms.
 3. The device of claim 1, wherein the silicondioxide layer is a monolayer.
 4. The device of claim 1, wherein thesilicon dioxide layer is terminated with ammonium groups.
 5. The deviceof claim 1, wherein 1%-10% of silicon atoms at a surface of the silicondioxide layer are terminated with ammonium groups.
 6. The device ofclaim 1, wherein the high dielectric constant material is halfniumoxide.